1. Field of the Invention
The invention relates to a control system and method for an internal combustion engine that performs learning of a throttle opening degree, at which a target idle speed can be obtained.
2. Description of Related Art
In an internal combustion engine such as that mounted on a vehicle, learning is performed to acquire a throttle opening degree, at which a target idle speed can be obtained. In the case where the state of combustion is temporarily deteriorated, for example because of occurrence of a misfire, during idle operation, the throttle opening degree at which the target idle speed can be obtained increases. Therefore, if learning is performed while the state of combustion is deteriorated, the learned value of the throttle opening degree may become larger than that for normal operation. In this case, while the learned value of the throttle opening degree is increased once, the learned value of the throttle opening degree is returned to the value for normal operation through re-learning when the deteriorated state of combustion is resolved.
Japanese Patent Application Publication No. 2010-014031 (JP 2010-014031 A) discloses a technique that enables continuation of learning of a throttle opening degree, even during deteriorated combustion, by relaxing the conditions on engine speed variations related to execution of learning control of the throttle opening degree when deterioration in the state of combustion is found.
In recent years, meanwhile, hybrid vehicles that include two driving power sources, namely an internal combustion engine and an electric motor, and vehicles that perform idle stop control to automatically stop an internal combustion engine during idling have been put into practice. In such vehicles, the frequency of idle operation is low, and hence there are fewer chances to learn a throttle opening degree during idle operation. Therefore, once the learned value of the throttle opening degree is varied by deterioration in combustion, there may be no chance for re-learning, and the value acquired during deteriorated combustion may be used continuously thereafter.
Such an issue can be suppressed to some degree by prohibiting learning, securing a longer learning period so that learning is performed not only based on a deteriorated state of combustion, or the like when the state of combustion is deteriorated. In such cases, however, it may take more time to complete learning, which may deteriorate the fuel efficiency.
The above issue may likewise occur in internal combustion engines other than those mounted on hybrid vehicles and vehicles that perform idle stop control if there are fewer chances to learn a throttle opening degree during idle operation.